Verbseoul.
하다 verb (adj)
미성숙하다miseongsukhadato be immature

To say to be immature politely in the present tense, use 미성숙해요 (mee-suhng-soo-kae-yoh). Below is the full conjugation of 미성숙하다 across the three tenses and three everyday speech levels.

Conjugation table

TenseCasual (반말)Polite (요)Formal (합니다)
Present
미성숙해
mee-suhng-soo-kae
미성숙해요
mee-suhng-soo-kae-yoh
미성숙합니다
mee-suhng-soo-kahm-nee-dah
Past
미성숙했어
mee-suhng-soo-kae-ssuh
미성숙했어요
mee-suhng-soo-kae-ssuh-yoh
미성숙했습니다
mee-suhng-soo-kae-sseum-nee-dah
Future
미성숙할 거야
mee-suhng-soo-kahl- -guh-yah
미성숙할 거예요
mee-suhng-soo-kahl- -guh-yae-yoh
미성숙할 겁니다
mee-suhng-soo-kahl- -guhm-nee-dah

Other useful forms

Command Polite (세요)
미성숙하세요
mee-suhng-soo-kah-sae-yoh
Suggestion / Let's Polite (요)
미성숙해요
mee-suhng-soo-kae-yoh
Question Polite (요)
미성숙해요?
mee-suhng-soo-kae-yoh-?
Connective “and” — (고)
미성숙하고
mee-suhng-soo-kah-goh
Conditional “if” — (면)
미성숙하면
mee-suhng-soo-kah-myuhn
Noun form — (기)
미성숙함
mee-suhng-soo-kahm